Metallic gold is conventionally extracted from gold-containing ores by dissolution in cyanide solutions. During the extraction process, large quantities of liquid wastes containing appreciable concentrations of cyanide, thiocyanate, and inorganic contaminants are produced. To protect the environment and to comply with governmental regulations, it is generally necessary to treat wastewater effluents for removal of toxic inorganic and cyanide constituents prior to discharge of wastewater effluents to the environment. Wastewater effluent from numerous other industrial processes likewise requires treatment for removal or reduction of inorganic constituents.
Retention of wastewater in tailings ponds is the oldest, and still a common method for treatment of gold mill wastewater effluents. Wastewater retention in a tailings pond promotes separation and settling of particulate contaminants. Many metals are soluble in wastewater as a result of cyanide complexing and contribute substantially to high levels of inorganic contamination. Retention of wastewater in tailings ponds additionally promotes photodegradation of both free and metallo-cyanides contained in the wastewater upon exposure to ultraviolet light. It has been suggested that aeration of wastewater collected in a tailings pond promotes contact of soluble cyanide with atmospheric carbon dioxide and converts soluble cyanide contaminants to a volatile form (HCN) which is eliminated from the pond. Although wastewater retention in tailings ponds may be effective to provide reduced levels of contamination, it generally cannot provide wastewater purification sufficient for direct discharge. Severe climatic conditions seriously impair the efficacy of tailings ponds, since natural degradation, photodegradation, precipitation and volatilization of contaminants cannot occur when the pond is frozen. Alternative methods for removal of contaminants must therefore be adopted where retention in tailings ponds is impractical or unfeasible, or it provides incomplete purification.
Ores containing significant amounts of arsenopyrite (generally in excess of 1% As) are not readily amenable to cyanidation. Arsenopyrite containing ores are typically concentrated by flotation and roasted during the milling process prior to cyanidation. Roasting releases oxidized forms of arsenic and sulfur constituents from the ore, and it produces wastewater and residues having high levels of arsenic contaminants. Mill wastewater effluent containing high levels of soluble arsenic is typically discharged to tailings retention ponds. Residues containing high levels of arsenic may be processed in arsenic reclamation plants or otherwise processed for recovery of soluble contaminants. Process water, cooling water and other process wastes associated with contaminant recovery are also discharged to tailings retention ponds and contribute significantly to contaminant arsenic levels, particularly in wastewater. Recent increases in gold mining activity, depletion of high purity gold ores, and stricter environmental regulations have resulted in heightened interest in processes for removal of cyanide, arsenic and other inorganic contaminants from wastewater generated during gold milling and other industrial operations. Effective effluent treatment providing removal of soluble contaminants, particularly cyanide and arsenic contaminants, permits mining of ores which contain high levels of arsenopyrite and other contaminants. Providing accessibility to lower grade ores by providing economically feasible methods for partitioning contaminants is increasingly important as sources of higher grade ores are rapidly being depleted.
Several methods for removal of cyanide from gold mill effluents have been proposed and implemented. The alkaline chlorination process for the destruction of cyanide involves oxidation of cyanide by the hypochlorite ion at a basic pH. Liquid chlorine or solid calcium hypochlorite typically provides the source of hypochlorite ion. The Inco SO.sub.2 /Air process utilizes mixtures of SO.sub.2 --O.sub.2 to promote oxidation of cyanide constituents in the presence of a soluble copper catalyst and under basic pH reaction conditions. Hydrogen peroxide has also been used as an oxidizing agent for removal of cyanide constituents in conjunction with a soluble copper catalyst. Biological removal of cyanide in a two stage digestion process has also been proposed. Acidification/volatilization/reneutralization processes based upon the volatility of the hydrogen cyanide produced when cyanide solutions are acidified have been developed. Cyanide removal by adsorption on ferrous sulfide has also been utilized, requiring Fe:CN ratios of at least about 3:1.
Removal of soluble arsenic from milling wastewater is also important where ores contain appreciable amounts of arsenopyrite. Conventional processes utilize ferric sulphate to provide ferric oxide and/or hydroxide particulates in the wastewater solution for precipitation of solubilized arsenic from wastewater by adsorption. Conventional processes for removal of soluble arsenic contaminants by adsorption on ferric particulates can be quite costly due to the chemical reagent requirements. In addition, treatment of wastewater containing elevated levels of arsenic using conventional ferric adsorption processes may result in a dramatic reduction in wastewater throughput and unacceptable arsenic removal levels and efficiencies.
Many of the processes described above for removal of cyanide and arsenic contaminants which have been implemented on a commercial scale are described in "State-of-the-Art of Processes for the Treatment of Gold Mill Effluents", J. Ingles and J. S. Scott, Mining, Mineral and Metallurgical Processes Division, Industrial Programs Branch, Environmental Protection Programs Directorate, July 1985.
U.S. Pat. No. 4,566,975 teaches a method for arsenic removal including at least two stages, wherein a precipitation agent comprising ions capable of forming insoluble hydroxide precipitates is added during a second or later process stage, and the solids precipitated are separated and returned to the first precipitation stage. Ferric sulphate may be introduced to first and second precipitation tanks, and sludge comprising arsenic and other contaminants adsorbed on ferric hydroxide particulates may be recycled from the second precipitation stage to the first precipitation stage, or to a premixing stage. Sludge is separated from liquids after first and second stage precipitation, but sludge is not separated from liquids after the preliminary premixing stage. The precipitation stages are carried out under basic reaction conditions at an elevated pH of about 8 to 9. Japanese patent publication J6 0125-292A teaches removal of soluble arsenic compounds from wastewater by adding a ferric (Fe III) compound to the effluent and adjusting the pH of the mixture to about 6-9 to coprecipitate ferric hydroxide and the complex formed between Fe(III) and arsenic-containing anionic complexes. The improvement comprises adding the Fe(III) compound to the effluent in the presence of sludge, thereby reducing the amount of Fe(III) compound which must be added to precipitate the solubilized arsenic.
U.S. Pat. No. 4,622,149 teaches an improvement to the Inco SO.sub.2 /Air process which contemplates addition of effective ferric ion in an amount of about three times the weight of the total arsenic and/or antimony content in the effluent. The teachings of the '149 patent describe use of the effective ferric ion in the second process stage in combination with the standard Inco SO.sub.2 /Air process wherein effluent is treated with SO.sub.2 and oxygen in the presence of soluble copper to produce treated effluent having a low inorganic contaminant content.
British Patent Specification 1,502,775 teaches removal of arsenic from acidic aqueous solutions by treatment with an arsenic precipitant (lime) in the first process stage to remove the bulk of solubilized arsenic, followed by treatment of the supernatant liquid with ferric or ferrous ion salts and an excess of oxidizing agent in a second process stage to complete arsenic precipitation. The oxidizing agent appears to be added contemporaneously with or subsequent to addition of the ferric ion salt.
U.S. Pat. No. 4,366,128 teaches removal of soluble arsenate from a solution at an elevated pH by adding a soluble barium salt to precipitate the arsenic as barium arsenate. U.S. Pat. No. 4,201,667 teaches removal of solubilized arsenic by addition of lime in the presence of phosphorus and oxidation of remaining arsenic constituents by addition of chlorine or hypochloride. U.S. Pat. No. 4,241,039 teaches removal of arsenic from acidic solutions wherein ferrous ions in solution are oxidized in the presence of oxygen under pressure and sulfuric acid. U.S. Pat. No. 4,025,430 teaches precipitation of metal ions and removal of hydroxides by addition of a soluble silicate solution. Japanese patent document J5 7150-478 teaches precipitation of solubilized arsenic using a ferric salt and if necessary, an oxidizing agent, and subsequently contacting the solution with an ion exchanger composed of amphoteric metal oxide hydrate to adsorb contaminants. Greek Patent Document SU 0710985 teaches precipitation of solubilized arsenic by iron-containing compounds followed by a biological digestion process.
Sabilization of solid arsenic trioxide (As.sub.2 O.sub.3) generated during roasting by chemical conversion to ferric arsenate is discussed in "Production of Environmentally Acceptable Arsenites-Arsenates from Solid Arsenic Trioxide," M. Stefanakis, A. Kontopoulis, Arsenic Metallurgy Fundamentals and Applications. p.287, Proceedings of Symposium sponsored by TMS-AIME Physical Chemistry Committee and Mackay Minerals Research Institute, 1988 TMS Annual Meeting. Oxidation of arsenic in solution by addition of hydrogen peroxide was followed by addition of ferric sulfate. Experimental results indicated that the stability of iron arsenate precipitate is satisfactory provided the molar Fe:As ratio associated with the iron arsenate precipitate is maintained at about 2.0 or above, and that basic pH reaction conditions generally result in increased arsenic solubility.
U.S. Pat. No. 4,724,084 teaches a process for removal of toxic organic materials and metals from wastewater containing high levels of organic and metal contaminants. The '084 patent teaches a two stage treatment process wherein ferrous sulfide is introduced to the effluent prior to addition of hydrogen peroxide. The admixture is clarified by pH adjustment with lime, and separation of particulates is facilitated by addition of a flocculating agent. After sludge removal, a second similar process stage is conducted.
U.S. Pat. No. 4,680,126 relates to removal of non-ferrous metals from wastewater by selective precipitation of ferrous metal ions followed by precipitation of the non-ferrous metals. U.S. Pat. No. 4,606,829 teaches removal of complexed zinc-cyanide from wastewater and involves a sludge recycle feature including aeration of the sludge to improve sludge stability and oxidize precipitated ferrous hydroxide. U.S. Pat. No. 4,343,706 teaches removal of heavy metals by flocculation with ferric ions at a basic pH. U.S. Pat. No. 4,321,143 teaches a process for reducing the COD content of aqueous waste by treatment with hydrogen peroxide in the presence of transition metal compounds and thereafter subjecting the waste to conventional biological degradation.
Although many of the prior art methods described above for removing soluble inorganic and cyanide constituents from wastewater are effective for removing substantial quantities of soluble inorganic and cyanide constituents, the cost of operating many of these wastewater treatment processes may be prohibitively high. Adsorption of soluble arsenic on insoluble ferric particulates, referred to hereinafter as the "ferric adsorption process," is an attractive process because it does not require complex, specialized equipment or extensive manual supervision. In general, however, as the level of soluble inorganic and cyanide contaminants increases, the quantities of chemical reagents required for contaminant removal and thereby the cost, increases correspondingly. Many of the prior art wastewater treatment processes are directed to removal of one or a single class of contaminants from wastewater, and multiple treatments may be required for substantially complete purification of wastewater.
Accordingly, it is an objective of the present invention to provide a process for removal of soluble inorganic contaminants, and particularly arsenic, from wastewater which demonstrates improved overall contaminant removal and process efficiency.
It is another objective of the present invention to provide an improved process for removal of substantially all soluble arsenic from wastewater, which is capable of treating wastewater having high levels of arsenic and other inorganic contaminants at a high rate of throughput.
It is still another objective of the present invention to provide an improved process for removal of arsenic and other contaminants from wastewater which is adaptable for treating wastewater having a broad range of contamination levels without requiring modification of plant equipment or design.
It is yet another objective of the present invention to provide an improved wastewater process which provides removal of free and complexed cyanide contaminants as well as inorganic contaminants such as arsenic, in an integrated, simplified treatment process.
It is still another objective of the process of the present invention to provide an improved process for converting soluble arsenic in wastewater to insoluble compounds and/or complexes which are stable over a wide range of reaction conditions.
It is yet another objective of the present invention to provide an improved process for removal of soluble arsenic and other inorganic contaminants from wastewater which is useful as a pretreatment stage for existing wastewater treatment facilities to provide improved overall contaminant removal and process efficiency.
It is still another objective of the present invention to provide an improved process for removal of solubilized inorganic and/or cyanide contaminants from wastewater which is easily implemented without involving substantial equipment, chemical, energy or supervisory requirements.